1. Field of the Invention
The present invention relates to a communication system. More particularly, the present invention relates to an apparatus and method for transmitting/receiving Channel Quality Information (CQI) in a communication system.
2. Description of the Related Art
The development trend of communication systems is toward providing high-speed large-capacity data services to Mobile Stations (MSs). To provide high-speed large-capacity data services, communication systems require higher data throughputs and thus use adaptive transmission schemes.
When using adaptive transmission schemes, a Base Station (BS) determines coding and modulation schemes for data to be transmitted to an MS according to the channel quality between the BS and the MS. A prevalent channel adaptive transmission scheme is Adaptive Modulation and Coding (AMC). When using AMC, a BS adaptively selects Modulation and Coding Scheme (MCS) levels for data to be transmitted to MSs according to the channel qualities between the BS and the MSs. For the selection of an adaptive MCS level, the BS should have knowledge of the channel qualities of the MSs. Hence, the MSs feed back CQIs representing the channel qualities and the BS determines MCS levels for the MSs based on the CQIs. The CQIs can be Signal-to-Interference and Noise Ratios (SINRs) or MCS level values, for example.
There are two types of CQI feedback, namely average CQI feedback and band CQI feedback. An MS transmits the average CQI of subchannels allocated to it in the former feedback scheme, whereas the MS transmits the CQIs of respective subchannels allocated to it in the latter feedback scheme. That is, the MS feeds back slow fading information about an allocated channel in the average CQI feedback scheme, and it feeds back fast fading information about the allocated channel in the band CQI feedback scheme. As described above, since the MS feeds back one average CQI regarding the allocated subchannels to the BS, the average CQI feedback has reduced CQI feedback overhead, compared to the band CQI feedback. Accordingly, it is preferable to use the average CQI feedback, when overhead in the system is an issue.
Despite the use of the average CQI feedback scheme, however, the CQI feedback overhead increases with the number of MSs serviced in the communication system. Moreover, there is inevitable overhead as the MSs each consume power to feedback their CQIs. Accordingly, there exists a need for a technique for reducing overhead such as CQI feedback-caused power consumption on the part of an MS, while reducing CQI feedback overhead on the part of a communication system.